In general, a carrier for use in mobile communications lies mainly within a certain band of high frequencies, such as a microwave band. On the other hand, signal processing in modulation and demodulation is carried out in a baseband in many cases in order to cut down on the cost. For this reason, base station apparatus need a circuit for converting data received in a high-frequency band into a signal in a baseband or in an intermediate frequency band. A device which functions as a band limiting filter is usually used for this frequency conversion. Such a band limiting filter is called an analog band pass filter. Since a band limiting filter is not a filter which can ideally limit the band of data passing therethrough, like a root Nyquist filter, intersymbol interference occurs in the data passing through the band limiting filter and therefore the data becomes distorted.
In order to correct for a signal which becomes distorted after passing through an analog band pass filter, there can be provided a digital filter having a band limiting characteristic which is inverse to the band limiting characteristic of the analog band pass filter with respect to an ideal filter in a base band signal processing unit. This digital filter is so constructed as to have a plurality of taps and to multiply the plurality of taps by different coefficients, respectively, and the coefficient values are chosen so that a portion in the band limiting by the analog band pass filter, which is different from the ideal band limiting, can be brought close to the ideal band limiting.
As one of prior art digital filters which are used for purposes relatively close to the above-mentioned purpose, there is a method disclosed in, for example, the following patent reference 1.
Although this method includes the step of correcting for degradation in the characteristics of an analog band pass filter due to a group delay using a digital filter disposed as a subsequent stage, the purpose of the method differs from that of a method in accordance with the present invention of correcting for degradation in the band limiting characteristic which occurs in a case of nonlinear input having a reception power level (or a received amplitude level). However, since the prior art method uses a similar means for correcting for degradation in the characteristics of an analog band pass filter using a digital filter, the prior art method will be introduced hereafter.
In the structure disclosed in this patent reference 1, a frequency conversion of a high-frequency signal received by an antenna element into a received signal having an intermediate frequency is carried out using a mixer and an analog band pass filter, a digital conversion of the received signal on which the frequency conversion has been performed into a digital signal is carried out using an A/D converter, and demodulation processing is then performed on this digital received signal by a demodulating unit. A digital filter is included in this demodulating unit, and corrects for distortion which has occurred in the signal because the analog band pass filter differs from an ideal filter such as a root Nyquist filter. The combination of the analog band pass filter and the digital filter is thus set up to provide band limiting close to such as ideal band limiting provided by a root Nyquist filter.
In addition, in the structure disclosed in this patent reference 1, an input signal and an output signal (or an A/D converted output signal) of the analog band pass filter are extracted, the characteristics of the group delay caused by the analog band pass filter are estimated using an algorithm, such as an LMS (Least Mean Squares: least square average) or an RLS (Recursive Least Squares), and a control signal is outputted to the digital filter so that a tap coefficient to be used by the digital filter is selected. Therefore, this prior art method offers an advantage of being able to correct for distortion which occurs in the signal using an optimal tap coefficient in which a group delay frequency characteristic is stored at all times. However, as a circuit which estimates the characteristics of the group delay is adapted to estimate the group delay characteristics more quickly, the circuit increases in its circuit size. On the other hand, as the circuit is downsized, the speed of estimating the group delay characteristics is reduced. When the power level (or amplitude level) of the received signal deviates from a linear region in which the characteristics of the components including the mixer, the analog band pass filter, and the A/D converter are taken into consideration, normal control cannot be performed on the digital filter according to the prior art method disclosed in patent reference 1. That is, the digital filter's adverse effect of increasing the distortion caused by the analog band pass filter instead of decreasing the distortion cannot be prevented.
In a structure disclosed by the following patent reference 2, a test signal is generated, data from a subsequent stage disposed behind a digital filter is outputted by a demodulating unit, an error rate is determined, and the tap coefficient of the digital filter is changed one by one so that the error rate is minimized. As a result, the digital filter whose error rate is the smallest is always selected during demodulation processing using a combination of an analog band pass filter and the digital filter used for correction of the analog band pass filter. However, it takes much time for the technique for minimizing the error rate while changing the tap coefficient of the digital filter to offer a guarantee that the digital filter goes into effect. Furthermore, when the power level (or amplitude level) of the received signal deviates from a linear region in which the characteristics of components including a mixer, the analog band pass filter, and an A/D converter are taken into consideration, the digital filter's adverse effect of increasing the distortion caused by the analog band pass filter instead of decreasing the distortion cannot be prevented.    [Patent reference 1] JP,2002-141821,A    [Patent reference 2] JP,11-122311,A
Since prior art wireless apparatus are constructed as mentioned above, distortion which occurs due to causes other than the fact that the analog band pass filter differs from an ideal filter, such as a root Nyquist filter, becomes dominant when the power level (or amplitude level) of the received signal is too small or too large to such an extent that the linearity of the power level cannot be maintained. A problem with prior art wireless apparatus is that in such a case, the use of the digital filter further degrades the band limiting characteristic as compared with a case where no digital filter is used.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a wireless apparatus which can prevent a digital filter thereof from further degrading the band limiting characteristic thereof when the power level (or amplitude level) of a received signal is too small or too large to such an extent that the linearity of the power level cannot be maintained.